This invention relates to a tone signal forming device employing a system for forming a tone signal having desired tone color characteristics in accordance with tone signal forming parameters such as a system for forming a tone signal by a filter control using a digital filter, a tone synthesis system using a frequency modulation operation and a tone synthesis system using an amplitude modulation operation and, more specifically, to realizing timewise change in the tone color in such tone signal forming device.
As a tone signal forming device for forming a tone signal of a desired tone color by a filter control using a digital filter, there is known a device disclosed in Japanese Patent Application Laid-Open No. Sho 62-127899 (corresponding to U.S. Pat. Application Ser. No. 934,781 now U.S. Pat. No. 4,841,828). In realizing timewise change in the tone color in this device, plural sets of different filter coefficients are stored in a memory and these coefficients are read out sequentially with a lapse of time and supplied to the digital filter.
Similarly, in other tone synthesis systems using tone forming parameters, plural sets of different tone forming parameters are stored in a memory and these parameters are sequentially read out with a lapse of time to form a tone in accordance with the read out parameters.
However, for continuously changing filter coefficients or other tone forming parameters one after another during sounding a tone which lasts a long time, considerable number of filter coefficients or other tone forming parameters must be stored in a memory and this necessitates a memory of a considerably large capacity. In view of economic considerations and the size of the device, there is a limit to the number of parameters which can be prepared with a result that it is difficult to realize adequate timewise change in the tone color for a tone such as a sustain tone which has a relatively long sounding time.